A legacy beamforming technique using multiple antennas can be mainly classified into an analog beamforming technique and a digital beamforming technique depending on a location to which a beamforming weight vector/precoding vector is applied.
First of all, the analog beamforming technique is a typical beamforming technique applied to an initial multi-antenna structure. The analog beamforming technique divides an analog signal of which digital signal processing is completed into a plurality of paths and sets phase-shift (PS) and power amplification (PA) to each of a plurality of the paths to perform beamforming. FIG. 1 is a diagram for an example of a legacy analog beamforming technique.
Referring to FIG. 1, it is able to see a structure that an analog signal derived from a single digital signal is processed by a power amplifier and a phase shifter connected to each antenna to perform analog beamforming. In particular, the phase shifter and the power amplifier process complex weights at an analog stage. In this case, a radio frequency (RF) chain corresponds to a processing block in which a single digital signal is converted into an analog signal.
However, according to the analog beamforming technique, since accuracy of a beam is determined according to characteristics of elements of the phase shifter and the power amplifier, the analog beamforming technique is advantageous for the narrow band transmission due to the control characteristics of the elements. On the other hand, since the analog beamforming technique has a hardware structure incapable of implementing multi-stream transmission, multiplexing gain for increasing a transmission rate is relatively small and it is difficult to perform beamforming per user based on orthogonal resource allocation.
Unlike the analog beamforming technique, the digital beamforming technique performs beamforming at a digital stage using a baseband process to maximize diversity and multiplexing gain in MIMO environment.
Referring to FIG. 2, digital beamforming can be performed by performing precoding in a baseband process. Unlike FIG. 1, an RF chain includes a power amplifier. This is because a complex weight derived for beamforming is directly applied to a transmission data.
And, since the digital beamforming technique is able to differently perform beamforming according to a user, it is able to support multi user beamforming at the same time. Since the digital beamforming technique is able to perform independent beamforming according to a user to which an orthogonal resource is allocated, the digital beamforming technique has characteristics that scheduling flexibility is high and it is able to manage a transmitting end in accordance with a system purpose. Moreover, when a technique such as Multiple-Input Multiple-Output (MIMO)-OFDM (Orthogonal Frequency Division Multiplexing) is applied in a broadband transmission environment, it may form an independent beam for each subcarrier. Therefore, the digital beamforming technique can maximize the maximum transmission rate of a single user based on increased system capacity and enhanced beam gain.
In accordance with the aforementioned advantages and disadvantages, digital beamforming-based MIMO technology has been introduced in the current 3G/4G system.